Listen to Catfish's recordings. Pay attention to the nuances of his playing. What parts of the beat does he emphasize? Recognize how often or how little he plays and when. Listen to how his parts interact with other instruments.
Master the 9th chord. Catfish lived on the 9th chord on many famous James Brown recordings. To play an E9, place your middle finger on the seventh fret of the A string. Set your index finger on the sixth fret of the D string. Lay your ring finger across the G, B and high E strings on the seventh fret. Practice this until all the notes ring cleanly.
Get into the groove. Hit the 9th chord while listening to the drummer. Accent the first beat of every measure, and strum on the "E-AND" of the beat (imagining 1-E-AND-AH 2-E-AND-AH). If you can't practice with a drummer, use a metronome.
Color the 9th chord by sliding up from having your middle and ring fingers on the sixth fret to the seventh when you are playing an E9. Alternatively, you can start the same chord one fret above the E9 (middle and ring fingers on the eighth fret) and slide down. You can also add some funk by hitting the high E string two frets above the 9th chord with your pinky. Combine all of these techniques together.
Plug in a chorus and delay pedal. Catfish loved effects, especially the chorus and echo pedals. He employed these at liberal settings rather than using them to subtly color the sound.
Plug in a chorus and delay pedal. Collins enjoyed using effects to color his sound. Chorus and delay were two of his favorites, and he mixed them in liberally rather than employing them subtly.